The mechanism of stone fragmentation with a pulsed solid state laser is known in the field of lithotripsy. The primary mechanism when using infra-red lasers in lithotripsy is usually of a photo thermal nature, wherein the thermal destruction of the stone composition with direct light absorption and minimal pressure waves is the predominant effect. For a maximum effect the laser fiber needs to be contact or close to the calculus which should be treated. A typical laser pulse of a flash lamp pumped solid state laser, which is conventionally used in the field of lithotripsy, usually creates a large, fast circular growing vapor bubble. While this growing vapor bubble has not reached the stone, energy from the pulse is still absorbed in the water. Once the vapor bubble has met the stone, the energy emitted by the pulse after that event is in the best case not absorbed by water before it reaches the stone and can completely be absorbed by the calculus. When the vapor bubble collapses the stone is moved away from the fiber tip due to the Kelvin Impulse. In addition, the calculus also moves due to the ablation plume ejected because of the conservation of the momentum. The expressions calculus and stone are used interchangeably in this text.
It is shown e.g. in “Dependence of Calculus Retropulsion on Pulse Duration During Ho:YAG Laser Lithotripsy” by Kang HW, et al. that the retropulsion could be reduced by applying pulses with lower amplitude, but longer pulse times. However, Kang also showed that the longer pulses reduce the ablation volume.
As described before, in many state of the art methods only a small part of the pulse energy is used for the stone disintegration. According to the state of the art in some cases the retropulsion is reduced while also reducing the ablation efficiency.
Further documents of the background for this invention comprise the document U.S. Pat. No. 5,820,627 by Rosen et al. disclosing real time optical feedback control of laser lithotripsy, wherein the laser pulse parameters are adjusted according to measured incandescent photoemissions emitted for irradiated biological material. The laser pulses are directed to a target area of the subject using a delivery system.
In the document WO 94/23478 titled “Q-switched laser system, in particular for laser lithotripsy” by Muller et al. a Q-switched laser system is described which has a laser active medium in a resonator, an optical pumping arrangement and a passive Q-switch. In addition, a resonator extension having an optical wave guide is associated to the laser-active medium in order to increase the laser pulse length.
Document WO 89/10647 discloses a device for generating laser pulses of variable duration. It discloses a device comprising a resonator in which a first continuously pumped neodymium-YAG (Nd:YAG) crystal and a switchable optical seal which functions as a Q-switch are arranged. To increase the pulse power in the beam path of the continuous-wave Nd:YAG laser, a second Nd:YAG crystal which is pumped in pulses is arranged after the resonator and outside the seal.
Further information about the background of this invention can be found for example in “Recoil momentum at a solid surface during developed laser ablation” (Quantum Electron 1993; 23:1035-1038 by Kuznetsov LI) as well as in “Comparison of the effects of absorption coefficient and pulse duration of 2.12 μm and 2.79 μm radiation on ablation of tissue” (IEEE J Quantum Electron 1996;32:2025-2036 by Frenz M, Pratiso H, Konu F, et al.). Further background documents include “Non contact tissue ablation by holmium YSGG laser pulses in blood” (Laser Surg. Med. 1991;11:26-34 by Van Leeuwen TG, van der Veen MJ, Verdaasdonk RM, Borst C.). Additional information can also be found in “Transient cavities near boundaries” (J Fluid Mech. 1986; 170:479-479 by Blake JR, Taib BB, Doherty G.).
Document WO 2008/024 022 A1 discloses a laser device based on two laser emitters. Document RU 2 272 660 C2 discloses a method of treating patients suffering from nepholythiasis.
Starting from this background, this invention addresses the problem of generating laser pulses which can then be used e.g. to increase the efficiency in methods and systems usable in lithotripsy. The invention is therefore also related to the question of how to generally improve a treatment method and system used for lithotripsy. The invention provides a method according to claim 1 and a lithotripter according to claim 9. Preferred embodiments are disclosed in the dependent claims.